Pigments are finely divided particulate solids which for use are typically dispersed in vehicles or on a substrate, such as inks, paints, or polymeric materials. Pigments may be organic or inorganic, and are usually unaffected by the vehicle or medium in which they are incorporated. They may alter appearance by selective absorption or by scattering light, and are often used as an integral part of decorative, protective and functional coatings. Pigments may be colored, colorless, black or white. Some pigments are corrosion inhibitors, fungistats, and/or antistatic agents. In general, pigments are insoluble solids and remain insoluble when dispersed in a vehicle, as contrasted with dyes, which are soluble.
Pigment particles may vary in size, shape and surface area. In commercial applications, typical particles are from 0.01-10.0 .mu.m in diameter and range from 1-1100 m.sup.2 /g in surface area. To achieve complete dispersion of a pigment in a vehicle, the surfaces of the pigment particles must be properly wetted.
Extender pigments are colorless or white pigments, generally with a refractive index less than 1.7. One example of an extender pigment is kaolin, a hydrous aluminosilicate mineral used as a filler in the paper and paperboard industry, and in paper coatings. As one example, pigmented paper may have a thin kaolin or other pigment coating. Calcined kaolins are white and hard, and are used for example in paper and in water based paints. Other extender pigments include clays, calcium carbonates, talc (magnesium silicates), and titanium dioxide.
Finely divided white mineral fillers may be added to improve optical and physical properties of paper sheets, primarily by filling the spaces between fibers. This produces a denser, softer, brighter and more opaque sheet, and may reduce cost because many fillers are less expensive than fibers. Clay is the most common filler, while talc is noted for its softness. Calcium carbonate is generally brighter than clay but can be used only in neutral or alkaline systems because of its solubility at lower pH levels. Titanium dioxide provides higher brightness and superior opacity, but is relatively expensive and inefficient. For example, as much as fifty percent of the titanium dioxide added to papermaking stock may be lost as waste, probably through formation of filler particles too large to remain on the paper.
Structured pigments are those which have been structurally modified, usually by thermal or hydrothermal chemical reactions. The most common structured pigment is calcined clay. Other structured pigments include aggregates comprising particulate matter treated with complex functional gels. A structured aggregate pigment of this type is described in Kaliski, et al., U.S. Pat. No. 5,116,418.
In papermaking, a variety of extenders, pigments, fillers, and dyes my be used to furnish or enhance specific sheet properties. These and other additives are often applied to wet paper stock during the manufacturing process, and are commonly referred to as "wet end" additives. Thus, "wet end retention," or the ability of an additive to adhere to wet stock, is an important operational parameter. An optically efficient pigment is readily retained, and has a high wet end retention, expressed as a percent of total pigment used.
At least two measures of pigment retention are in common use. One is overall retention, which is the percent of total pigment added to the stock that is retained by the paper. The second measure is single-pass retention, and is the percent of pigment retained by the paper after a single pass or application.
Overall retention is important economically, because any additive which is not retained is lost, or must be recovered and recycled. Single-pass retention is a measure of sheet quality and affects paper machine operation. Pigments with a low single-pass retention require additional recycling (i.e. repeated applications of pigment) and frequently result in a non-uniform pigment distribution. Low retention can cause an uneven distribution in the cross-section of the sheet, and may produce different surface properties on the two sides of the sheet, a phenomenon known as "two-sidedness." Undesirable buildups and agglomerations may also occur in the paper machine.
Percent first pass retention can be calculated according to the following formula: ##EQU1## where Cf is the % consistency (solids content) of the furnish or papermaking stock and Cw is the % consistency of the whitewater, or waste runoff. The first pass retention of a particular pigment or filler in a papermaking furnish can be calculated by the "Percent first pass ash retention" method, according to the formula: ##EQU2## where Af is the % filler ash of the furnish and Aw is the % filler ash of the whitewater. The ash retention, also discussed below, is the ratio of the amount of ash in the paper to the total amount of ash in the papermaking system, expressed as a percent. The ash content is determined by filtering, burning, and weighing steps, to separate the pigment from the other papermaking materials.
Two basic mechanisms for retention have been recognized: absorption and filtration. Larger particles tend to be retained by filtration; the particles become enmeshed in and bound by the paper-making fibers during the manufacturing process. Smaller particles are retained by absorption. Factors affecting retention include the amount, particle size, shape and density of the fillers, the order in which different materials are introduced to the paper stock, and the ionic balance of the paper stock components. Retention is also affected by the conditions in the paper machinery and at the paper-forming wire. Temperature, pH, type of pulp fibers, sheet weight, wire mesh size, type of dewatering, degree of system closure, and machine speed can all affect pigment retention.
Retention can be measured by making the paper and measuring its optical properties, or by using a Britt Jar, which controls various parameters and is close to manufacturing conditions. See, e.g. Example 1.
Separate retention aids and fixing agents have been developed to improve the wet end retention of pigments and other additives. However, these chemicals often result in coagulation (or flocculation) and must be used with care. Coagulating chemicals such as alum (aluminum sulfate) or polymers are added to form gelatinous precipitates or flocs, which absorb and enmesh the pigment. This can be difficult to control, and can affect the quality and surface characteristics of the paper. Thus, there is a need for pigments having a high wet end retention, without resort to fixing agents.
Known retention aids are often designed to affect the ionic balance of colloidal particles, including pigments, in the papermaking stock. Particles suspended in a liquid tend to have an electrical charge, and are surrounded by a dense layer of ions also having a specific electrical charge. This layer in turn is surrounded by a more diffuse charged layer, and the bulk liquid also has an electrical charge. The difference in electrical charge between the dense layer of ions and the bulk liquid is the zeta potential, usually measured in millivolts. In general, retention of colloids in a papermaking system tends to improve as the zeta potential approaches zero. Pulp fibers and filler colloids tend to be anionic, or negatively charged. Retention agents balance that charge with positive ions, such as (Al.sup.3+) supplied as Al.sub.2 (SO.sub.4).sub.3 or alum. Alum can neutralize negatively charged fiber and pigment colloids to zero zeta potential, and the resulting equilibrium improves wet end retention. However, the alum can also form an aluminum polymer which bridges from particle to particle, causing significant flocculation: large coagulants of ionically attracted particles can form.
Efforts have also been made to modify the anionic nature of papermaking fibers themselves. To this end, cellulose fibers have been reacted with materials that impart a positive charge to the fibers, for example by introducing cationic nitrogen atoms to side chains of the cellulose molecules. Known cationic pulp fibers are described in Gess, U.S. Pat. No. 4,624,743. Gess describes in particular a cellulose pulp product that is treated under alkaline conditions with a condensate of epichlorhydrin and dimethylamine, to produce a cationic product. These fibers are described as having improved anionic dye retention generally, and improved pigment retention under some conditions. Gess notes in particular (column 13 line 3) that his treatment is not a universal means for cationizing papermaking additives, and will not provide improved cationization or retention for all papermaking systems.
Other known cationic additives include cationic starch, and cationic kaolin clay. For example, cationized kaolin is described in European Patent No. 0382427 and in Nemeh et al, U.S. Pat. No. 4,767,466. Another known filler is cationic titanium dioxide, disclosed in Savino, U.S. Pat. No. 4,874,466. Savino discloses a filler formed of a papermaking pigment (e.g. titanium dioxide) mixed with a cationic water soluble polymer having at least 50% repeating units of quaternary ammonium salts. The quaternary material may be a copolymer of epichlorhydrin and dimethylamine. According to Savino, the cationic polymer increases the opacity and retention of titanium dioxide by promoting electrostatic attraction between anionic sites in the paper and the cationic charge imparted to the filler.
While some paper products might be made with cationic fibers, or using cationic agents and fillers, there remains a need for pigments which have improved wet-end retention, without requiring special papers or separate additives. Novel cationic or positively charged pigments having improved wet end retention are now provided here. These pigments overcome flocculation problems associated with known additives, and are not limited to the use of specific pulps or papermaking stocks. The first pass retention of these new cationic pigments approaches 100%, and is markedly higher than for normal (untreated) pigments. Moreover, these new pigments can be made and used more economically than known retention aids or cationic pulp systems.
As one example, a structured aggregate pigment comprising a particulate material and a functional microgel, and available commercially as Norplex 604 (Nord Kaolin Company, Jeffersonville, Ga.) has a first pass retention of 4% in a conventional Britt Jar system. When treated according to the invention, the resulting pigment has a first pass retention of 94%.